Role of Dissolved Gas in Ionic Liquid Electrolytes for Secondary Lithium Metal Batteries

Abstract

Lithium metal is a potential anode in lithium batteries; however, high cycling efficiency has not been achieved. The effect of dissolved gas on the reversibility of a Li/Li+ electrode in an ionic liquid electrolyte was investigated. The ionic liquid electrolyte was saturated with argon, nitrogen, oxygen, air, or carbon dioxide, and the Coulombic efficiency for the reduction and reoxidation of lithium metal was measured. Secondary ion mass spectroscopy (SIMS) was used to determine the thickness and composition of the solid electrolyte interphase (SEI) layer formed on the lithium metal. The Coulombic cycling efficiency was highest with oxygen, nitrogen, and air. The Coulombic efficiency was significantly lower with dissolved carbon dioxide. SIMS revealed that the SEI layer was made up of primarily LiF regardless of the gas used. The thickness of the SEI layer was the main determinant of cycling efficiency. A thinner SEI, observed with nitrogen and oxygen, lead to the highest Coulombic efficiency, whereas carbon dioxide produced a thick SEI that appeared to inhibit lithium deposition.